Liquid scintillation counters are commonly used for measuring the count rate or activity of samples containing low energy beta particles or corresponding particles emitting radionuclides such as tritium and carbon-14.
The range of the low energy beta particles in the sample is generally few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with the scintillation medium, which comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the solutions. In this interaction process most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle. These scintillation photons are detected usually by two, in coincidence operating, photomultiplier tubes producing electric pulses. The height of the pulses are proportional to the amount of emitted scintillation photons and thus proportional to the energy of the interacted beta particle.
When measuring sample activities with liquid scintillation counters, the basic problem is the reduction of the counting efficiency due to the quenching of the sample.
It is known in the liquid scintillation counting that the reduction of the counting efficiency due to the quenching of the sample can be corrected by a means of a quench curve which describes the relationship between the counting efficiency and the amount of quench of the sample.
Normally liquid scintillation counters are provided with one detector and they are designed to measure samples in 7 ml or 20 ml glass or plastic vials.
A novel liquid scintillation counter, which counts samples, directly from multi-well sample plates is described in International Patent Publication Number WO 89/12838 (Lehtinen et al.). The apparatus counts liquid scintillation or corresponding samples directly from sample plates which comprise several separate sample wells or vials. The apparatus has one or several detectors in order to count one or several samples at a time. The sample plate is placed in the counting position or pre-counting position manually or automatically on a rigid plate holder made of photon attenuating material and having holes for the wells of the sample plate. The walls of the holes are reflecting or scattering in order to guide the photons from the liquid scintillation sample to the detectors, built of two photomultiplier tubes operating in coincidence and situated on the opposite sides of the holes of the plate holder. The wells of the sample plate can be closed by an adhesive transparent tape. The apparatus can be used also for counting gamma radiation emitting samples if the holes of the sample plate are surrounded by gamma radiation sensitive detectors.
Another novel scintillation counting system for in-situ measurement of radioactive samples in a multiple-well plate is presented under European Patent Publication Number 0425767A1 (VanCauter et al.). This apparatus is provided with multiple photomultiplier tubes positioned adjacent to the sample wells containing the scintillator for simultaneously measuring the radioactivity of multiple samples with only a single photomultiplier tube sensing the scintillations from each well and converting the sensed scintillations into corresponding electrical pulses. The electrical pulses from each photomultiplier tube are processed to discriminate between pulses attributable to sample events within the wells and pulses attributable to non-sample events such as photomultiplier tube noise. The discrimination is effected by determining whether a selected number of electrical pulses occurs with a prescribed time interval, the occurrence of the selected number of pulses within the prescribed time interval signifying a sample event. Only the electrical pulses attributable to sample events are supplied to a pulse analyzer.
The multi-well sample plates have typically eight rows of wells, whose diameter is 7-8 mm arranged in twelve columns with d distance of 9 millimeters between the center points of the wells. The typical volumes of the sample wells of such 96-well sample plates are 200-400 microliters depending on the height of the plate. When the wells of the multi-well sample plate are separate, the plate can be placed for counting on a rigid sample plate holder made of photon attenuating material and having thru-holes for the wells of the sample plate as shown in the patent application published under international publication number WO 89/12838. As a consequence, an optically isolated compartment is formed around each sample well of the sample plate. Unfortunately most of the commercially available multi-well sample plates are transparent and the wells are joined together with ribs or the like in order to stiffen the sample plate. As a consequence of this it is impossible to isolate the wells optically. As a further consequence of this some amount of the scintillation photons produced by the absorption of the beta particle in a certain sample well may travel to other sample wells and thus produce an undesired increase in observed count rates in those wells. This phenomenon is called optical crosstalk. It is known that the use of opaque multi-well sample plates can reduce optical crosstalk as mentioned in TopCount Topics PAN0005 6/91, published by Packard Instrument Company, Meriden USA 1991. Unfortunately in many applications transparent multi-well sample plates are preferred, and most of the commercially available multi-well sample plates are transparent.
U.S. Pat. No. 4,348,588 (Yrjonen et al.) shows a crosstalk correction method applied in gamma counting. This method is not suitable for liquid scintillation counting because it does not take into account the variation of the quench of the samples, and it is limited to correcting only affection of samples, which are in pre-determined locations.